1. Field of the Invention
The present invention generally relates to a fuel vapor treatment apparatus for an internal combustion engine.
2. Background Information
Internal combustion engines are sometimes provided with a fuel vapor treatment apparatus or system having a canister that temporarily adsorbs fuel vapor generated inside the fuel tank. When the engine enters prescribed engine operating conditions, the adsorbed fuel vapor is separated and mixed with air to form a purge gas. A purge control valve opens to direct the purge gas to a purge passage that feeds the purge gas into an intake system of a fuel system while controlling the flow rate of the purge control valve. As a result, evaporation of fuel vapors into the atmosphere is prevented. Generally, the opening and closing of the purge control valve to control the flow rate of the purge gas is typically duty controlled. One example of such a fuel vapor treatment apparatus is disclosed in Japanese Laid-Open Patent Publication No. 5-215020.
In recent years, more stringent regulations regarding fuel vapor evaporative emissions have led to fuel vapor treatment apparatuses that use large capacity canister with increased purge rates (quantity of fuel vapor purged per unit of time).
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved fuel vapor treatment apparatus that improves the purge control performance of a purge control valve. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.
It has been discovered that when larger purge control valves are used to satisfy the aforementioned demand for increased purge rates, the sudden change in flow rate of the purge gas is large when purging is started at a low purge gas flow rate. Moreover, the use of a large purge control valve can result in an increase in air-fuel ratio fluctuations when a low purge gas flow rate control is executed during idling and other times when the intake air flow rate is small. As a result, it is easy for poor operating performance to occur in using such fuel vapor treatment apparatuses.
In view of the aforementioned problems with the prior art, one object of the present invention is to provide an internal combustion engine fuel vapor treatment apparatus that is durable and can eliminate the sudden change in flow rate that occurs when purge control starts, even when a large capacity purge control valve is used.
The forgoing object can basically be attained by providing a fuel vapor treatment apparatus for an internal combustion engine that basically comprises a purge control valve, an operating condition detector, a purge gas flow rate setting component and a control unit. The purge control valve is configured to open and close a purge passage that introduces purge gas containing fuel vapor into an air intake system of the engine to control a purge gas flow rate of the purge gas. The operating condition detector is configured to detect at least one engine operating condition. The purge gas flow rate setting component is configured to set the purge gas flow rate of the purge gas quantity to be supplied to the air intake system based on the engine operating condition detected by the operating condition detector. The control unit is configured to output a duty value and a drive frequency to the purge control valve to duty control the opening and closing of the purge control valve. The control unit sets a high drive frequency during a low flow rate control of the purge gas and sets a low drive frequency during a high flow rate control of the purge gas.
These and other objects, features, aspects and advantages of the present invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses a preferred embodiment of the present invention.